Silicone rubber composition yielding a structural ash on combustion

ABSTRACT

A SILICONE RUBBER FORMULATION INCORPORATING SODIUM METASILICATE AND AN INORGANIC COMPOUND, PREFERABLY NICKELOUS OXIDE, HAVING A MELTING PONT EXCEEDING THAT OF SODIUM METASILICATE. ON CATALYTIC VULCANIZATION OF THE FORMULATION, A RUBBER-LIKE COMPOSITION RESULTS WHICH ON COMBUSTION YIELDS A COHESIVE, RIGID CERAMIC MATERIAL.

3,772,239 Patented Nov. 13, 1973 US. Cl. 260 -37 SB 2 Claims ABSTRACT OFTHE DISCLOSURE A silicone rubber formulation incorporating sodiummetasilicate and an inorganic compound, preferably nickelous oxide,having a melting point exceeding that of sodium metasilicate. Oncatalytic vulcanization of the formulation, a rubber-like compositionresults which on combustion yields a cohesive, rigid ceramic material.

This is a continuation of application Ser. No. 749,578,

filed Aug- 2, 1968, now abandoned.

. This invention relates to a silicone rubber formulation vulcanizableto yield a composition capable on combustion of converting to a ceramicmaterial functional as a thermal barrier.

Although silicone rubbers are well known for their inherent resistanceto high temperatures, at least as compared to organic rubbers bothnatural and synthetic, because of the demands of certain applications,much effort has been expended toward increasing this resistance. For themost part, this effort has been in the direction of additives, such asorganic halo compounds which are known fortheir fire retardantproperties and hence tend to reduce the rate of burning and/ or thevolume of rubber consumed incident to the burning.

Conceding that these additives are reasonably effective, there stillremains a temperature at which the silicone rubber, as, otherwiseconventionally prepared, will convert to an ash totally withoutstructural strength or integrity and incapable of functioning to anyextent as the original rubber or of affording any other beneficialeffect.

The present invention is based on a concept altogether different fromthat of prior workers. In accordance therewith, the heat of combustionof the vulcanized rubber composition itself is utilized to produce arigid, coherent material serving as a thermal barrier and constituting asubstitute forthe original composition in one or more significantrespects.

There are many areas in which the invention may be practically applied.Thus,'it finds application in the fabrication of seals and gaskets,particularly such of these as'are designed for use in oil refineries andchemical plants. It is' manifestly desirable inthe event of a fire atsuch an establishment that the gaskets between pipe Wall anddoor-forming members or the entire safe may be.

lined with such composition.

. Still another application of the invention resides in its employmentin, the compartmentalization of aircraft so as to contain fires whereveroriginating.

United States Patent Oihce The invention contemplates inter aliaarticles including a substrate on which the silicone rubber is carriedor with which it is integrated. Inch such a case, the rubber may be ineither a cured or uncured state at the time it is brought intoassociation with the substrate, which commonly is of a fibrous nature,e.g., fiber glass, asbestos, etc.

A composition conforming to the invention is distinguished in that itcomprises, in admixture with a silicone gum, which may be a gel orviscous liquid, sodium metasilicate and an inorganic compound having ahigher melting point than sodium metasilicate. The applicable inorganiccompounds are otherwise characterized in that they are either naturallyoccurring or derivable from naturally occurring minerals. They may thusbe identified as inorganic mineral compounds. In the milieu of theinvention, such compounds serve in co-action with the sodiummetasilicate as densifiers and structuring agents. As exemplary thereofmay be mentioned: nickelous oxide (bunsenite), zinc oxide, cryolite,aluminum fluoride, manganese oxide, chromium oxide, titanium dioxide,calcined alumina, nepheline syenite, lithium aluminum silicate,fluorspar, etc. Of these, as previously indicated, nickelous oxide ispreferred.

The silicone gum with which the sodium metasilicate and higher meltinginorganic mineral compound are admixed may be filled or unfilled, i.e.,depending on the particular application the gum may or may notincorporate reinforcing and/ or non-reinforcing filler materials such asare conventionally employed in the silicone industry. If a reinforcingfiller is used, various additives may also be included in thecomposition as, for example, diphenylsilanediol, which is operative toprevent crepe aging.

In the practice of the invention, the intermixing of the sodium silicateand inorganic mineral compound with the gum, when a gel, is generallycarried out on a tworoll mill wit-h the rolls heated as required.

As a matter of economy, it is generally in order to employ in theformulation subjected to milling a substantial quantity of anon-reinforcing filler, such as finely ground quartz. The material soldunder the trade name Min-U-Sil is applicable here. It is to beunderstood that such material represents merely an inert extender andthat no criticality is attached thereto.

It is customary in the particular art to express the percentages of theseveral components of a silicone rubber formulation in parts by weightper 100 parts of the silicone rubber base. On this basis, it may be saidthat useful compositions conforming with the invention may be producedusing from 20 to 100 parts of sodium silicate and from 5 to 20, morepreferably 8 to 12, parts of the higher melting inorganic mineralcompound.

The reason for the stated preference of nickelous oxide as the highermelting inorganic component resides in the fact that the compositionproduced on combustion of the vulcanized'formulation is somewhatsuperior in point of rigidity and tendency to crack or craze. This isparticularly so where the maximum temperatures reached during thecombustion lie between the kindling temperature of the rubber and 1400F. The nickelous oxide provides excellent ceramic structures throughoutsuch range and these structures hold through temperatures upwards of5000 to 6000 F.

The catalytic vulcanization of the formulation including the sodiumsilicate and the higher melting mineral compound maybe effected inaccordance with conventional practice. Thus, a peroxide catalyst, suchas 2,4-

dichlorobenzoyl peroxide, is normally used. The catalyst V is best addedas a dispersion in a silicone fluid. Toprevent premature vulcanization,the dispersion should not be milled-in until the intermixing of theother materials (when carried out on hot rolls) is complete and therolls have cooled. The vulcanization of the mixture including thecatalyst can be carried out using conventional equipment.

It is to be understood that in addition to the sodium silicate andhigher melting mineral compound, the formulation subjected tovulcanization may include one or more additional ingredients which arefunctional. Thus, materials of the nature of fillers of various particlesize may be added to increase or decrease the porosity of the combustionproduct or to insure that the volume occupied by the combustion productwill be as required by the particular application.

Various aspects of the invention are illustrated by the examples below.These are not to be taken as in any way limitative of the invention.

EXAMPLE 1 In this experiment, a silicone rubber base of the followingformulation was employed:

Parts (A) A trimet-hylsiloxy end-blocked gum copolymer of 98 mol percentdimethylsiloxane units and 2 mol percent methylvinylsiloxane units(Williams plasticity=0.1l inch) 100 (B) A reinforcing fured silicafiller having an average surface area of 325 square meters per gram 40(C) 50 centipoise OH-terminated dimethylsiloxane fluid (for softening)13 (D) Diphenylsilanediol (antistructure additive) 2 100 parts of theabove base was milled with:

Parts (E) Anhydrous water-soluble sodium silicate powder (Na/Sil:l)capable of passing a ZOO-mesh screen 30 (F) Finely ground quartz 30 (G)Nickelous oxide (H) 2,4-dichlorobenzoyl peroxide dispersed in an equalportion by weight of a 1000 cs. silicone oil 2 Here a fluid siliconerubber gum was employed in order to obtain a flowable paste compound.The formula was as follows:

Parts (A) A trimethylsiloxy end-blocked gum copolymer of 98.5 molpercent dimethylsiloxane units and 1.5 mol percent methylvinylsiloxaneunits (312,-

000 centistokes) 100 (B) Sodium metasilicate powder capable of passing200 mesh 30 (C) Finely ground quartz 30 (D) Nickelous oxide 10 (E)2,4-dicl1lorobenzoyl peroxide dispersed in an equal portion by weight ofa 1000 cs. silicone oil 2 The above mixture was spread coated on a'wovenglass fabric to a thickness of .010 inch. This coated fabric was thenoven-cured for 30 minutes at 400 F. to obtain a flexible and conformableelastomer coated membrane. Further exposure to dry heat at 1400 F. gavea rigid glass fabric-reinforced structural ash occupying essentially thesame volume as the original rubber coated fabric.

EXAMPLE 3 A formula identical to that described in Example 1 wasemployed except that the finely ground quartz was omitted. Aftercombustion was completed at 1400 F., the volume of the residue was about/5 less than that of the composition fired.

EXAMPLE 4 Formulation A B C Cab-O-Sil, pts None 40 40 Soak or wet outprior to curing, days 7 7 None Each of the three specimens A, B, and Cwas press vulcanized to the elastomeric state at 240 F. into a A-inch x3-inch diameter disc, then fired at 1400 F. until combustion wascomplete. In the case of A, the residue was quite rigid and had theappearance of a shredded sponge with a very irregular surface. Itsvolume was approximately 10 percent greater than that of the originalcured disc. Minor stress cracking on the periphery occurred incident tothe cooling from 1400 F. to room temperature.

In the case of B, the volume of the rigid residue after 1400 F. wasapproximately equal to that of the original disc. Again, slight stresscracks around the periphery occurred while cooling to room temperature.The surface was smooth with good detail including the clearly definedsample number originally molded in during vulcanization of the specimen.

In the case of C, the fired residue occupied approximately 15 percentmore volume than the disc before firing. No stress cracking occurredwhile cooling and the surface was smooth and regular. Internalexamination of the fired specimen revealed a larger pore structure thanwas the case with A or B. The specimen exhibited high impact strength.

EXAMPLE 5 EXAMPLE 6 Other mineral compounds useful in place of, or inaddition to, nickelous oxide include zinc oxide, cryolite, aluminumtrifluoride, manganese oxide, chromium oxide, titanium dioxide, calcinedalumina, nepheline syenite, lithium aluminum silicate, and ceramicfiuorspar;

Any of these compounds contribute integrity in the ash during or afterexposure to temperatures between 500 F. and 1000 F. In this range,sodium silicate alone willnot cause the ash residue to rigidify andconsequently the mass is friable after combustion.

The invention claimed is: 1. A rubber-like composition which oncombustion is converted to a rigid material having substantialstructural integrity, said composition obtained from thee vulcanizationof a substantially solvent free heat curable composition containing asilicone gum copolymer consisting essentially of chemically combineddimethylsiloxane units, methylvinylsiloxane units and trimethylsiloxaneunits, a peroxide catalyst and from about 20 to about 100 parts byweight based on 100 parts by weight of the silicone gum of sodiummetasilicate and from about 5 to about 20 parts by weight based on 100parts by Weight of the silicone gum of an inorganic mineral compoundhaving a melting point higher than sodium metasilicate.

2. The composition of claim 1 where the inorganic mineral compound isnickelous oxide.

References Cited UNITED STATES PATENTS 11/1951 Simon et a1. 260-46.5 R7/1963 Bobear 260-37 SB HOSEA E. TAYLOR, Primary Examiner M. I. MARQUIS,Assistant Examiner US. Cl X.R.

1l746 R, 62; 25262; 260-465 G; 264'44

